Broadband light source device and method of creating broadband light pulses

ABSTRACT

A broadband light source device for creating broadband light pulses includes a hollow-core fiber and a pump laser source device. The hollow-core fiber is configured to create the broadband light pulses by an optical non-linear broadening of pump laser pulses. The hollow-core fiber includes a filling gas, an axial hollow light guiding fiber core configured to support core modes of a guided light field, and an inner fiber structure surrounding the fiber core and configured to support transverse wall modes of the guided light field. The pump laser source device is configured to create and provide the pump laser pulses at an input side of the hollow-core fiber. The transverse wall modes include a fundamental transverse wall mode and second and higher order transverse wall modes.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/875,699, filed May 15, 2020, which is a continuation of U.S.application Ser. No. 16/471,035, filed Jun. 19, 2019, which is aNational Phase Entry of International Application No. PCT/EP2017/000023,filed Jan. 9, 2017, which are hereby incorporated herein in theirentireties by reference.

FIELD

The present disclosure relates to a broadband light source device forcreating broadband light pulses, in particular to a broadband lightsource device including a gas-filled hollow-core photonic crystal fiberof non-bandgap type (HC-ARF) pumped by ultrashort laser pulses.Furthermore, the disclosure relates to a method of creating broadbandlight pulses, in particular by coupling ultrashort pump laser pulsesinto a HC-ARF and creating the broadband light pulses by an opticalnon-linear broadening of the pump laser pulses in the fiber.Furthermore, the disclosure relates to a hollow-core photonic crystalfiber, which is configured for creating broadband light pulses by anoptical non-linear broadening of ultrashort pump laser pulses in afilling gas within the fiber. Applications of the disclosure areavailable in particular in ultraviolet (UV) light based metrology, e.g.semiconductor metrology, and inspection.

BACKGROUND

In the present specification, reference is made to the following priorart illustrating the technical background of the disclosure:

-   [1] P. St. J. Russell et al. in “Nature Photonics” 8, 278-286    (2014);-   [2] U.S. Pat. No. 9,160,137 B1;-   [3] F. Tani et al. in “PRL” 111, 033902 (2013);-   [4] F. Belli et al. in “Optica” 2, 292-300 (2015);-   [5] N. M. Litchinitser et al. in “Opt. Lett.” 27, 1592-1594 (2002);-   [6] F. Gebert et al. in “Opt. Exp.” 22, 15388-15396 (2014);-   [7] P. Uebel et al. in “Opt. Lett.” 41, 1961-1964 (2016); and-   [8] J. C. Travers et al. in “J. Opt. Am.” B 28 (2011), A11-A26.

It is generally known that optical semiconductor metrology or materialinspection systems rely on bright light sources, typically emittingbroadband radiation from the vacuum or deep ultraviolet (UV) to the nearinfrared (IR). Light source architectures are often based on thegas-discharge effect, i.e. by generating an electric discharge in anionized gas (plasma). A disadvantage of these sources can be theirintrinsic spatial incoherence, resulting from a very high number ofoptical modes within the volume of the discharge arc. For a metrologyapplication a well-defined illumination path is required which requiresrelatively complex optics that makes access of the sample difficult. Inaddition, focusing to a diffraction-limited spot requires spatialfiltering, resulting in loss of most of the spectral power.

As an alternative, multiple lasers at different wavelengths or whitelight laser sources have been suggested. In the latter case, broadbandpulsed radiation within a range spanned from deep UV to near IR isgenerated by optically pumping a filling gas within a hollow-coreoptical fiber. The spectral conversion from ultrashort pump laser pulsesto the broadband fiber output is the result of non-linear processes, inparticular modulational instability, soliton fission and dispersive wavegeneration (see [1], [2], [3] and [4]).

Hollow-core optical fibers are typically divided into two classesdepending on the physical guidance mechanism: hollow-core photonicbandgap fibers (HC-PBFs) and hollow-core anti-resonant-reflecting fibers(HC-ARFs, fibers of non-bandgap type). White light laser sources requirethe use of HC-ARFs, which have a sufficient broadband transmissionwindow for guiding the broadband pulsed radiation.

The light guiding mechanism in HC-ARFs, in particular HC-ARFs of Kagomeor single ring type, as illustrated in FIGS. 6 and 7 (prior art, notconsidering the generation of new frequencies), is mostly based onanti-resonant reflection of light from the walls surrounding thehollow-central core (see e.g. [5]). This two dimensional confinementleads to the formation of transverse core modes and the anti-resonanceallows for relatively broadband guiding windows.

FIG. 6A shows a cross-sectional view of the HC-ARF of Kagome type asdescribed in [6], wherein guiding of the core modes and the creation ofwall modes of light guided within the walls have been investigated for arange of core wall thicknesses from about 190 nm to 295 nm. As analternative, the HC-ARF of single ring type, as shown in FIG. 6B, hasbeen described in [7], wherein the effects of the single ring diameter,the inner core diameter and the wall thickness t on the fibertransmission have been investigated.

A disadvantage of the mentioned HC-ARFs results can be seen in theeffect of resonances of the core modes and the wall modes on thetransmission of the HC-ARF, as illustrated in exemplary manner in FIGS.7 and 3A (curve A2). The transmission through a piece of e.g. 60 cmsingle ring fiber with a wall thickness t=0.44 μm shows first (m=1, at0.92 μm) and second (m=2 at 0.46 μm) core wall resonances, that arevisible in the fiber transmission spectrum as transmission dips.

As a further disadvantage, it has been found in [6], where thetransmission of narrow-band UV light (single-wavelength at e.g. 280 nm)through the HC-ARF has been investigated, that a transmissiondegradation may occur just after a few hours of operation (similar asshown in FIG. 3B (curve B2)). In [6], it has been suggested that thetransmission loss may result from fabrication-induced variations in thethickness of the walls surrounding the core. Furthermore, it has beensuggested that the transmission degradation of continuous wave lightfields can be suppressed if certain wall thicknesses in the range fromabout 190 nm to about 290 nm are provided and thickness variations areavoided.

Furthermore, transmission degradations have been identified in [6] asthe result of core-wall-resonances, wherein the wavelengths where thecore mode phase-matches to wall modes can be approximated bykh _(cw)√{square root over (n _(g) ²(λ)−n _(m) ²)}=qπ, q=1,2, 3 . . .wherein n_(m) is the mode index (refractive index of the filling gas,about 1), n_(g) is the refractive index of the fiber material, h_(cw) isthe thickness of the single rings and the positive integer q defines thetransverse resonance order supported by the fiber walls. It has beenfound in [6] that by selecting the narrowband wavelength of the guidedlight, a transmission degradation can be avoided.

With the application of HC-ARFs for white light generation, e.g.according to [1] or [8], and an appropriate adjustment of pump pulse andbeam parameters, fiber structure, filling gas type and pressure, abroadband output pulse signal in particular in the UV range (wavelengthtypically below 350 nm) can be generated and guided to the fiber end.However, due to the above transmission degradation in HC-ARFs, theapplication of the fibers for UV generation can be limited.

SUMMARY

The objective of the disclosure is to provide an improved broadbandlight source device and an improved method of creating broadband lightpulses, being capable of avoiding or reducing disadvantages ofconventional techniques. In particular, the broadband light is to becreated with improved efficiency, reduced transmission loss in the fiberand/or increased long term stability of operation.

These objectives are solved with a broadband light source device, amethod of creating broadband light pulses or a hollow-core fiber,comprising the features of the independent claims, respectively.Preferred embodiments and applications of the disclosure are defined inthe dependent claims.

According to a first general aspect of the disclosure, the aboveobjective is solved by a broadband light source device for creatingbroadband light pulses, comprising a hollow-core fiber of non-bandgaptype (hollow-core anti-resonant-reflecting fiber) and a pump lasersource device.

The hollow-core fiber is any type of non-bandgap hollow light guidingfiber (HC-ARF) being adapted for accommodating a filling gas and forcreating the broadband light pulses by an optical non-linear broadeningof pump laser pulses. The hollow-core fiber has an axial hollow lightguiding fiber core including the filling gas, e.g. a noble gas, like Ar,Ne, He, Kr, Xe, a Raman-active gas like H₂, D₂, N₂ or a gas mixture, andan inner fiber structure. The hollow light guiding fiber core supportscore modes of a guided light field. The inner fiber structure has innerwalls extending and surrounding the fiber core along the longitudinalextension of the hollow-core fiber, and it supports transverse wallmodes of the guided light field. The transverse wall modes include afundamental transverse wall mode and second and higher order transversewall modes. The pump laser source device is arranged for creating andproviding a periodic sequence of ultrashort pump laser pulses (pumplaser pulses having a duration below 1 ps) at an input side of thehollow-core fiber.

Preferably, the pump laser source device is adapted for providing sub-pspulses with a high repetition rate, e.g. above 100 Hz. The repetitionrate of the pump laser source device depends on the choice of the pumpsource. For example, Ti:Sapphire based pump sources typically operate at1 kHz, while fiber based pump sources can operate from single shot totens of MHz.

The sequence of light pulses created in the fiber have a core modespectrum being determined by a fiber length, a fiber core diameter, atleast one pump pulse parameter and/or beam parameter of the pump laserpulses and at least one gas parameter of the filling gas. In particularthe spectral range covered by the core mode spectrum is determined thefiber length and the fiber core diameter. Pump pulse parameters comprisee.g. at least one of pulse duration, pulse energy, pulse shape and pulsespectrum. Beam parameters e.g. at least one of modal shape, pointing andstability of the laser beam (light field) provided by the sequence ofpump laser pulses.

According to the disclosure, the inner fiber structure of thehollow-core fiber is configured such that at least the second and higherorder transverse wall modes of the transverse wall modes and the coremode spectrum have a spectral displacement relative to each other. Inother words, by designing the inner fiber structure of the hollow-corefiber at least the second and higher order resonance positions arespectrally displaced to the generated light confined in the core modes.Accordingly, there is a spectral gap between at least the second andhigher order transverse wall modes and the spectral range covered by thecore mode spectrum. The transverse wall modes do not overlap with thespectral distribution of the core modes.

According to a second general aspect of the disclosure, the aboveobjective is solved by a method of creating broadband light pulses,wherein pump laser pulses are directed into a hollow-core fiber ofnon-bandgap type including a filling gas and the broadband light pulsesare created by an optical non-linear broadening of the pump laser pulsesin the hollow-core fiber. The hollow-core fiber supports transverse wallmodes and a core mode spectrum, which is determined by a fiber length, afiber core diameter, at least one pump pulse parameter and/or beamparameter of the pump laser pulses and at least one gas parameter of thefilling gas.

According to the disclosure, at least second and higher order transversewall modes of the hollow-core fiber are spectrally displaced relative tothe core mode spectrum of the hollow-core fiber. Accordingly, a HC-ARFis used, which is designed for providing the spectral displacement ofthe wall modes relative to the core mode spectrum. Preferably, thebroadband light laser pulses are created with the broadband light sourcedevice according to the first general aspect of the disclosure.

Advantageously, the inventive broadband light source device and methodare capable of synthesizing broadband, high brightness radiation. Inparticular, the emitted fiber output (broadband light pulses, alsodesignated as UV-IR pulses) has a spectrum covering at least part of theUV wavelength range. Preferably, the emitted spectrum is included in arange from the deep-ultraviolet (UV), e.g. 250 nm to the near-infrared(IR), e.g. 1100 nm. The emitted spectrum is free of spectral featuresdetermined by resonances of core modes with wall modes. Furthermore, theterm “broadband light source device” refers to a system being adaptedfor creating a pulsed output included in this emitted spectrum. Thebroadband light source device can be configured as a table-top device,and it can be used as a tool e.g. for applications in optical metrology(in particular in semiconductor applications), spectroscopy or lifesciences. The disclosure provides a broadband light creationperformance, which might be compared to conventional silica-core,fiber-based supercontinuum systems but extends the emitted spectrum tothe deep UV. Compared to conventional broadband lamps, the emitted beamis spatially coherent resulting in a dramatically increased spectralbrightness and the fiber output results in superb beam pointingstability.

In particular, the disclosure is based on the following considerationsby the inventors. Firstly, at the resonance positions, where the coremodes and the wall modes have equal or similar propagation parameterswithin the fiber, the UV radiation is strongly leaking out of the fibercore and experiences strong attenuation before reaching the fiber end.As a consequence, for providing a strong UV signal to the end user, theresonances and an increase of the transverse field distribution comparedto the relatively confined core modes are to be avoided. Secondly, if UVportions of the broadband light strongly overlaps with the inner wallswhich are part of the inner fiber structure and typically made fromsilica, gradual fiber degradation is likely to occur (attributed tosolarisation). This can be avoided by a suppression of leaking lightinto the walls.

Accordingly, as an essential advantage, with the inventive separation ofthe core modes and at least the second and higher order transverse wallmodes, resonant coupling of light from the core modes to the transversewall modes is suppressed, resulting in an increased confinement of thecore modes, an overall flatter generated spectrum and a longer lifetime.

Furthermore, the leakage of light field power to the inner structure ofthe fiber is reduced. The inventors have found that changes in the fiberwall material, in particular the formation of light absorbing sections,can be minimized by reducing the power load to the fiber, so that anincreased long term stability of operation can be obtained.

Accordingly, compared to existing HC-ARF based schemes for broadbandlight generation in the ultraviolet spectral range, the disclosureincreases the lifetime allowing long term and stable generation of lightin the ultraviolet spectral region with strongly reduced fiberdegradation. In addition, the resonance reduced nature of the fibers inthe range of the generated spectrum increases the spectral flatness ofthe emitted broadband signal and the same time results in a uniformtransverse mode profile.

The above advantage can be obtained with the inventive decoupling of thecore modes and the second and higher order transverse wall modesalready. This results from the following theoretical considerations bythe inventors. The fundamental resonance occurs at longer wavelengthsthan the higher-order resonances. As a consequence, solarisation is lesslikely to occur, because multi-photon-absorption is needed. Furthermore,the overlap integral between the core mode and the fundamental wallmodes is different compared to higher-order resonances. As aconsequence, less energy may be transferred from the core mode to thewall modes. Finally, the spectral bandwidth in which energy transferoccurs is different and dependent on the slope difference of theeffective index of the wall modes with respect to the core modes. Forthe fundamental resonance, the slope difference is shallower resultingpotentially in a larger spectral bandwidth.

However, if according to a preferred embodiment of the disclosure, alltransverse wall modes, i.e. the fundamental, second and higher ordertransverse wall modes, and the core mode spectrum have a spectraldisplacement relative to each other, the complete suppression ofresonant coupling of core and wall modes is obtained. Advantageously,the fiber structure is designed such that the first core wall resonancelies below the shortest wavelength of the generated spectrum. The fiberstructure used according to the disclosure minimizes the modal overlapwith the wall material, e.g. glass, in the UV, consequently preventsdamage of the fiber and thus further increases the lifetime of thesystem.

According to a further preferred embodiment of the disclosure, designingthe inner fiber structure of the hollow-core fiber, in particularHC-ARF, for decoupling of the core and wall modes is obtained byselecting a wall thickness of the fiber walls of the inner fiberstructure facing to the fiber core. The wall thickness is selected atleast along a downstream portion of the fiber to be below a limit wallthickness such that at least the second and higher order, preferably alltransverse wall modes are spectrally shifted to shorter wavelengthsrelative to the core mode spectrum. Advantageously, the wall thicknessis a fiber parameter which can be easily adjusted when manufacturing thehollow fiber or after manufacturing thereof (e.g. by etching). As asurprising result, the inventors have found that hollow fibers can beprovided with sufficiently thin wall thickness of inner structuresadjacent to the hollow core without impairing the fiber stability orsensitivity to the light field power guided in the fiber.

As an example, practical tests by the inventors have shown, thatreducing the wall thickness from e.g. 0.32 μm (conventional hollowfiber) to about 120 nm, the number of resonances in the transmissionspectrum (representing the coupling of core and wall modes) can bereduced from three to one at about 0.25 μm. Even with this remainingresonance in the long wavelength section of the broadband spectrum, asubstantial increase of the fiber operation duration could be obtained.

The hollow fiber can have a distribution of wall thicknesses along itslength, changing from a larger thickness at the input side to athickness below the thickness limit wall thickness towards the outputside. If the fiber walls of the inner fiber structure have the selectedwall thickness below the limit wall thickness exclusively in alongitudinal section of the hollow-core fiber where the broadband lightpulses are created and transmitted through the hollow-core fiber towardsthe output end thereof, further advantages in terms of fiber stabilityin an upstream portion of the fiber are obtained. Furthermore, it can bebeneficial if the fiber has relatively thick walls at the input side toreduce confinement loss of the pump.

The disclosure can be implemented with any type of the mentionednon-bandgap photonic fibers, including the hollow core and a regulararrangement of inner structures adjacent to the hollow core. Preferably,the inner fiber structure comprises a single ring or a Kagome structure,which have been subjected to extended investigations of their lightguiding properties. With the single ring or Kagomé type HC-ARF, thefiber walls facing to the fiber core preferably have a wall thickness(t) being selected such that

$\begin{matrix}{t < \frac{\lambda_{\min}}{2\sqrt{n_{2}^{2} - n_{1}^{2}}}} & (1)\end{matrix}$wherein λ_(min) is a shortest wavelength of the core mode spectrum, n₁is a refractive index of the filling gas in the hollow-core fiber and n₂is a refractive index of the inner fiber structure. Advantageously, theabove formula provides a limit wall thickness, which can be set independency on a few known parameters only.

Particularly preferred, the wall thickness is selected in a range from70 nm to 300 nm, in particular from 70 nm to 150 nm, e.g. with a hollowfiber made of glass (silica). The lower limit has been found to providesufficient mechanical stability to the hollow fiber and the innerstructure thereof. The upper limit advantageously provides suppressingresonances of the core and wall modes. With the upper limit 150 nm, thespectral separation from all wall modes is obtained. Furthermore, thehollow fiber preferably is adapted for supporting a core mode spectrumhaving the shortest wavelength λ_(min) in a range from 170 nm to 250 nm.

According to a further preferred embodiment of the disclosure, thebroadband light source device further includes an adjustment device,which is adapted for adjusting at least one of the at least one pumppulse and/or beam parameter. In a system for practical routine use, thepump laser source device might be preconfigured such that the correctpulse parameter is set. In this case, only control of the beam parameteris provided. Advantageously, the adjustment device is capable ofchanging e.g. the beam pointing of the pump pulses injected to thehollow fiber, i.e. the beam center relative to the fiber center.Particularly preferred, the adjustment device further includes a sectionbeing adapted for adjusting the at least one gas parameter of thefilling gas supplied from an optional gas supply device to the hollowfiber, like the gas pressure and/or the gas type.

According to another preferred embodiment of the disclosure, thebroadband light source device further includes a monitoring device beingarranged for monitoring at least a part of the core mode spectrum of thebroadband light pulses output from the hollow core fiber.Advantageously, the monitoring step allows an on-line measuring andtesting the created broadband light pulses. Particularly preferred acontrol loop is provided including the monitoring device and theadjustment device. The control loop is adapted for controlling theadjustment device such that the spectral displacement of the transversewall modes and the core mode spectrum is kept during creating thebroadband light pulses.

According to a third general aspect of the disclosure, the aboveobjective is solved by a hollow-core fiber, in particular a HC-ARF,which is adapted for creating broadband light pulses by an opticalnon-linear broadening of pump laser pulses in a filling gas. Thehollow-core fiber has an axial hollow fiber core, which is filled withthe filling gas and which supports core modes of a guided light field ofthe broadband light pulses, and an inner fiber structure, whichsurrounds the fiber core and which supports transverse wall modes of theguided light field. The transverse wall modes include a fundamentaltransverse wall mode and second and higher order transverse wall modes.The broadband light pulses have a core mode spectrum being determined byat least one pump pulse parameter of the pump laser pulses and at leastone gas parameter of the filling gas. According to the disclosure, theinner fiber structure of the hollow-core fiber is configured such thatat least the second and higher order transverse wall modes and the coremode spectrum have a spectral displacement relative to each other. Usingthe hollow-core fiber for creating broadband light pulses represents afurther independent subject of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

Further details and advantages of the disclosure are described in thefollowing with reference to the attached drawings, which show in:

FIG. 1 : a schematic view of an embodiment of a broadband light sourcedevice according to the disclosure;

FIGS. 2 and 3 : schematic illustrations of the inventive design of aHC-ARF;

FIG. 4 : a schematic view with further details of an embodiment of abroadband light source device according to the disclosure;

FIG. 5 : an output spectrum of broadband light pulses created with theinventive method; and

FIGS. 6 and 7 : schematic illustrations of conventional HC-ARF's andtransmission spectra thereof (prior art).

DETAILED DESCRIPTION

Features of the disclosure are described in the following withparticular reference to the broadband light pulse generation in an UVlight source device including an HC-ARF of Kagome or single ring typeand a control loop for adjusting in particular a pump source device anda gas supply device. The disclosure is not restricted to theseembodiments but rather can be implemented with other types of HC-ARFsand/or without the automatic loop control. Details of the opticallynon-linear processes for spectrally broadening the pump pulses in thehollow fiber are not described as these are known as such from priorart.

Aspect 1 of the description—Broadband light source device (100), beingconfigured for creating broadband light pulses (1), comprising:

-   -   a hollow-core fiber (10) of non-bandgap type including a filling        gas and being arranged for creating the broadband light pulses        (1) by an optical non-linear broadening of pump laser pulses        (2), wherein the hollow-core fiber (10) has an axial hollow        light guiding fiber core (11), which supports core modes of a        guided light field, and an inner fiber structure (12), which        surrounds the fiber core (11) and which supports transverse wall        modes of the guided light field, and    -   a pump laser source device (20) being arranged for creating and        providing the pump laser pulses (2) at an input side (13) of the        hollow-core fiber (10), wherein    -   the transverse wall modes include a fundamental transverse wall        mode and second and higher order transverse wall modes, and    -   the broadband light pulses (1) have a core mode spectrum being        determined by a fiber length, a fiber core diameter, at least        one pump pulse and/or beam parameter of the pump laser pulses        (2) and at least one gas parameter of the filling gas,

characterized in that

-   -   the inner fiber structure (12) of the hollow-core fiber (10) is        configured such that at least the second and higher order        transverse wall modes and the core mode spectrum have a spectral        displacement relative to each other.

Aspect 2 of the description—Broadband light source device according toaspect 1, wherein the inner fiber structure (12) of the hollow-corefiber (10) is configured such that all transverse wall modes and thecore mode spectrum have a spectral displacement relative to each other.

Aspect 3 of the description—Broadband light source device according toone of the foregoing aspects, wherein fiber walls (15) of the innerfiber structure (12) facing to the fiber core (11) have a wall thicknessbeing selected such that at least the second and higher order transversewall modes are spectrally shifted to shorter wavelengths relative to thecore mode spectrum.

Aspect 4 of the description—Broadband light source device according toaspect 3, wherein the fiber walls (15) of the inner fiber structure (12)have the selected limit wall thickness exclusively in a longitudinalsection of the hollow-core fiber (10) where the UV light pulses (1) arecreated and transmitted through the hollow-core fiber (10).

Aspect 5 of the description—Broadband light source device according toaspect 3 or 4, wherein the inner fiber structure (12) comprises a singlering or a Kagome structure, and the fiber walls (15) facing to the fibercore (11) have a wall thickness (t) being selected such that

$t < \frac{\lambda_{\min}}{2\sqrt{n_{2}^{2} - n_{1}^{2}}}$wherein λ_(min) is a shortest wavelength of the core mode spectrum, n₁is a refractive index of the filling gas in the hollow-core fiber (10)and n₂ is a refractive index of the inner fiber structure (12).

Aspect 6 of the description—Broadband light source device according toaspect 5, wherein the wall thickness is in a range from 70 nm to 300 nm,in particular from 70 nm to 150 nm, and/or the shortest wavelengthλ_(min) of the core mode spectrum is in a range from 170 nm to 250 nm.

Aspect 7 of the description—Broadband light source device according toone of the foregoing aspects, further including an adjustment device(30) being arranged for adjusting at least one of at least one pumppulse parameter, in particular pulse duration, pulse energy, pulse shapeand/or pulse spectrum, and at least one beam parameter, in particularmodal shape, pointing and/or stability.

Aspect 8 of the description—Broadband light source device according toaspect 7, further including a gas supply device (40) being connectedwith the hollow-core fiber (10) and being arranged for supplying thefilling gas to the hollow-core fiber (10), wherein the adjustment device(30) is connected with the gas supply device (40) for adjusting the atleast one gas parameter of the filling gas.

Aspect 9 of the description—Broadband light source device according toone of the foregoing aspects, further including a monitoring device (50)being arranged for monitoring at least a part of the core mode spectrumof the UV light pulses (1) output from the hollow core fiber.

Aspect 10 of the description—Broadband light source device according toaspect 9, further including a control loop (60) including the monitoringdevice (50) and the adjustment device (30), wherein the control loop(60) is adapted for controlling the adjustment device (30) such that thespectral displacement of the transverse wall modes and the core modespectrum is kept during operation of the UV light source device (100).

Aspect 11 of the description—Method of creating broadband light pulses(1), comprising the steps of:

-   -   coupling pump laser pulses (2) into a hollow-core fiber (10) of        non-bandgap type including a filling gas, wherein the        hollow-core fiber (10) has an axial hollow light guiding fiber        core (11), which supports core modes of a guided light field,        and an inner fiber structure (12), which surrounds the fiber        core (11) and which supports transverse wall modes of the guided        light field, wherein the transverse wall modes include a        fundamental transverse wall mode and second and higher order        transverse wall modes, and    -   creating the broadband light pulses (1) by an optical non-linear        broadening of the pump laser pulses (2) in the hollow-core fiber        (10), wherein    -   the broadband light pulses (1) have a core mode spectrum being        determined by a fiber length, a fiber core diameter, at least        one pump pulse and/or beam parameter of the pump laser pulses        (2) and at least one gas parameter of the filling gas,

characterized in that

-   -   at least the second and higher order transverse wall modes and        the core mode spectrum are spectrally displaced relative to each        other.

Aspect 12 of the description—Method according to aspect 11, wherein alltransverse wall modes and the core mode spectrum are spectrallydisplaced relative to each other.

Aspect 13 of the description—Method according to one of the aspects 11or 12, wherein fiber walls (15) of the inner fiber structure (12) facingto the fiber core (11) have a wall thickness being selected such that atleast the second and higher order transverse wall modes are spectrallyshifted to shorter wavelengths relative to the core mode spectrum.

Aspect 14 of the description—Method according to aspect 13, wherein thefiber walls (15) of the inner fiber structure (12) have the selectedlimit wall thickness exclusively in a longitudinal section of thehollow-core fiber (10) where the broadband light pulses (1) are createdand transmitted through the hollow-core fiber (10).

Aspect 15 of the description—Method according to one of the aspects 11to 14, wherein the inner fiber structure (12) comprises a single ring ora Kagomé structure, and the fiber walls (15) facing to the fiber core(11) have a wall thickness (t) being selected such that

$t < \frac{\lambda_{\min}}{2\sqrt{n_{2}^{2} - n_{1}^{2}}}$wherein λ_(min) is a shortest wavelength of the core mode spectrum, n₁is a refractive index of the filling gas in the hollow-core fiber (10)and n₂ is a refractive index of the inner fiber structure (12).

Aspect 16 of the description—Method according to aspect 15, wherein thewall thickness is in a range from 70 nm to 300 nm, in particular from 70nm to 150 nm, and/or the shortest wavelength λ_(min) of the core modespectrum is in a range from 170 nm to 250 nm.

Aspect 17 of the description—Method according to one of the aspects 11to 16, further comprising a step of adjusting at least one of the atleast one pump pulse and/or beam parameter.

Aspect 18 of the description—Method according to aspect 17, furthercomprising the steps of supplying the filling gas to the hollow-corefiber (10), and adjusting at least one of the at least one gas parameterof the filling gas.

Aspect 19 of the description—Method according to one of the aspects 11to 18, further including a step of monitoring at least a part of thecore mode spectrum of the broadband light pulses (1) output from thehollow core fiber.

Aspect 20 of the description—Method according to aspect 19, furtherincluding controlling the adjusting step with a control loop (60), suchthat the spectral displacement of the transverse wall modes and the coremode spectrum is kept during creating the broadband light pulses (1).

Aspect 21 of the description—Hollow-core anti-resonant-reflecting fiber(10), being arranged for creating broadband light pulses (1) by anoptical non-linear broadening of pump laser pulses (2) in a filling gas,wherein

-   -   the hollow-core fiber (10) has an axial hollow fiber core (11),        which is filled with the filling gas and which supports core        modes of a guided light field of the broadband light pulses (1),        and an inner fiber structure (12), which surrounds the fiber        core (11) and which supports transverse wall modes of the guided        light field, wherein the transverse wall modes include a        fundamental transverse wall mode and second and higher order        transverse wall modes, and    -   the broadband light pulses (1) have a core mode spectrum being        determined by a fiber length, a fiber core diameter, at least        one pump pulse parameter of the pump laser pulses (2) and at        least one gas parameter of the filling gas,

characterized in that

-   -   the inner fiber structure (12) of the hollow-core fiber (10) is        configured such that at least the second and higher order        transverse wall modes and the core mode spectrum have a spectral        displacement relative to each other.

FIG. 1 shows a schematic sketch of the inventive setup for broadbandUV-light generation. The broadband light source device 100 comprises ahollow-core fiber 10 and a pump laser source device 20. The hollow-corefiber 10 is e.g. an HC-ARF of single ring type, as shown with theenlarged cross-sectional illustration and in FIG. 2 , having a hollowcore 11 and an inner fiber structure 12 and extending along a straightlongitudinal direction from an input side 13 to an output side 14 of thehollow-core fiber 10. According to the cross-sectional illustration, theinner fiber structure 12 comprises a regular arrangement of e.g. sixthin-walled capillaries each with a wall 15 extending with tube shapebetween the input and output sides 13, 14. The hollow-core fiber 10 hasa length of e.g. 50 cm and a core diameter of e.g. 25 μm. The wall 15has a thickness t of e.g. 300 nm.

The pump laser source device 20 comprises e.g. a pulse source of thetype solid state or fiber laser emitting a sequence of pump pulses 2with a duration in a range from 5 fs to 1 ps, a center wavelength in arange from 200 to 2000 nm and a repetition rate in a range from 0.001kHz to 100 MHz.

The hollow-core fiber 10 is fixedly arranged in a gas supply device 40,which comprises a gas cell accommodating a filling gas, like e.g. Ar.The gas cell can be connected via a controllable valve with a gas source(not shown), and it has input and output windows 41, 42 transmitting thepump pulses 2 and the broadband light pulses 1, resp. The input andoutput windows 41, 42 are made of glass with optical quality. In apractical system for commercial use, the external gas source might notbe needed. For example, the gas cell can be filled with the filling gasduring production and sealed.

For creating the broadband light pulses 1, the beam of the pump laserpulses 2 is directed via the input window 41 onto the input side of thehollow fiber 10 and coupled into the hollow fiber core 11 thereof. Thepump laser pulses 2 are injected along a beam path being coincident withthe longitudinal axis of the hollow fiber 10. The hollow fiber 10supports core modes of the guided light field as illustrated inexemplary manner below with reference to FIG. 5 . Furthermore, the innerfiber structure 12 supports transverse wall modes of the guided lightfield. Within the hollow fiber the broadband light pulses 1 are createdby an optical non-linear broadening of the pump laser pulses 2, e.g.around the position 16. The broadband light pulses 1 have a broadbandcore mode spectrum which depends on pump pulse and pump beam parameters,the type of the filling gas and the density (pressure) thereof. The coremode spectrum is set and the hollow fiber is configured such that thetransverse wall modes and the core mode spectrum are spectrallydisplaced relative to each other.

According to the disclosure, the fiber structure is selected so that thefiber wall thickness t is given by the above equation (1). The minimumwavelength generated is an interplay between fiber structure and length,pump pulse and pump beam parameters and gas type and pressure(influencing the refractive index of the filling gas).

FIG. 2 shows another example of a hollow fiber 10, illustrated with ascanning electron microscope image. The single-ring hollow fiber 10 has150 nm (thin-walled) walls obtained from a conventional hollow fiberwith 360 nm walls by HF etching. The inset shows a close-up of the wall15.

By filling the hollow fiber 10 with a gas, like e.g. Ar, and adjustingthe pump parameters, the pump pulses are subjected to spectralbroadening and the measured spectrum of broadband light pulses is shownin FIG. 3A (curve A1). The measured spectrum (curved A1) does not showresonance dips in contrast to the output spectrum of a conventionalthick-walled fiber (curve A2), which clearly shows two pronounced dipsin the signal (m=1 around 760 nm and m=2 around 390 nm).

When the system is operated with a conventional fiber over severalhours, clearly a decay of the output power is visible (curve B2 in FIG.3B). The signal drops by about 20% in 0.6 Wh. If the fiber has thinwalls according to the disclosure the lifetime test (curve B1 in FIG.3B) shows that the degradation has been improved by a factor of morethan 300×.

FIG. 4 shows a block diagram with further details of the broadband lightsource device 100, which can be divided into an optical head 110, inwhich the broadband light pulses are synthesized and provided to theuser, and a control unit 120 that contains electronics, control of thepump source 20 and interfaces.

The optical head 110 includes discrete modules 111 to 115 for pumpsource 20, pre-processing, synthesizing and post-processing. All modules111 to 115 are integrated into a single enclosure on a mutual, robustbase plate to optimize stability. In a laboratory environment, theoptical head 110 is typically placed on an optical table and thebroadband light pulses 1 are emitted from the post-processing module115.

The pump source module 111 contains the pump source device 20 that emitssub-ps pulses with some tens μJ energies at a repetition rate betweenseveral 0.001 kHz to a few 10 MHz (adjustable by anelectronically-controlled modulator), resulting in up to a few 10 W ofaverage power. The pump source device 20 is typically a fiber orthin-disk laser operating at a central wavelength in the near-IR or thecorresponding harmonics (e.g. in the green or UV).

In the pre-processing module 112, pump pulse and pump beam parametersare monitored by optoelectronic means (input check). Additionally, themodule 112 includes an adjustment device 30, which shapes relevantproperties like beam stability, pulse energy, average power,polarization or beam diameter (beam control). Optionally, anelectronically-controlled shutter is inserted to prevent the pump pulsesfrom being delivered to the synthesizing module 113.

The synthesizing module 113 contains optical elements, includingmirrors, lenses and/or polarization optics to couple the pump pulses2—engineered by the pre-processing module 112—into the core of ahollow-core optical fiber (HC-ARF) 10. The optical elements forincoupling are mounted in selected holders and mechanics to optimizestability and coupling efficiency. The hollow fiber 10 is incorporatedinto one or more gas cells (se FIG. 1 ) that are connected to theoptionally provided fluid module 114. The gas cells are designed so thatthe hollow fiber 10 can be filled by a fluid (usually a noble gas likeAr, Ne, He, Kr, Xe, a Raman-active gas like Hz or a gas mixture). Usingseveral gas cells (e.g. at the in- and out-coupling sides of the hollowfiber 10) result either in a constant pressure distribution along thefiber, or—if different pressures are set—in a pressure gradient. Theends of a gas cell contain either a suitable window (see FIG. 1 ) totransmit the input/output pulses or a pressure-tight fitting to connectan additional cell.

The fluid module 114 includes a section of the adjustment device 30comprising an electronic pressure regulator with a range from low vacuumto several 10 bar. The module 114 furthermore may comprise interfaces toconnect via high pressure and vacuum lines 116 to a gas supply device40, including gas reservoirs and a vacuum pump (not shown).

Relevant parameters of the synthesized broadband light pulses 1, likee.g. average power, beam pointing stability, spectrum, beam quality ornoise, are monitored with a monitoring device 50 included in thepost-processing module 115. Feedback is given to the pre-processingmodule 112 to optimize coupling into the hollow fiber 10 (output check)and to the fluid module 114. In particular, a part or all of thesynthesized spectrum is monitored and instabilities in the signal can becompensated by the beam stabilization system of the pre-processingmodule 112. Such instabilities may be a consequence of mechanicalmisalignment due to stress or thermal effects. Additionally, thespectrum is flattened and provided to the end user through a window.

An important feature of the system is dynamic feedback technology,integrated into the post-processing module 115, which monitors part ofthe emitted spectrum and provides a feedback signal through a systemcontrol loop 60 including the control unit 120 to the beamstabilization, to optimize the UV synthesizing process.

The control unit 120 is divided into controls 121 to 123 for the pumpsource device 20, the beam stabilization (section of the adjustmentdevice 30), the gas supply (further section of the adjustment device 30)and the general system. In connection with the monitoring and adjustmentdevices 30, 50, the control unit 120 provides a control loop 60 forautomatic regulation of the device operation. The control unit 120 ismounted in a 19″ rack housing and the cables are long enough so that thecontrol unit can be placed up to several meters away from the opticalhead 110.

The pump source control 121 includes the electronics, opticsand—optionally—chiller to control the operation of the pump sourcedevice 20. With the beam stabilization control 122, an includedmicro-controller sets the performance of the beam stabilization systemin the pre-processing module 112 and optimizes coupling into the hollowfiber 10. The system control 123 contains several A/D converters andmicro-controllers to monitor and set various system parameters.Additionally this control 123 allows the user to interact with thesystem (standby- and on/off-switches) and provides different interfacesto connect the optical head 110 to the control unit 120 as well asinterfaces for external computer control (RS232 and/or USB).

In operation of the broadband light source device 100, the pump pulses 2are generated by the pump source module 111. The pump pulse beam is thendelivered in free space towards the pre-processing module 112. Here,commercially available electronic, optical and mechanical elements areused for input check and beam control. The free-space beam is coupledinto the core of the hollow fiber 10 and excites the transverse,fundamental core mode. Because of the pump pulse parameters (e.g. some100s fs pulse duration), the regime of modulation instability is (MI) isaccessed to spectrally broaden the pulse [8]. The spectrally broadenedoutput beam is collected by optical elements, shaped by thepost-processing module 115 using commercially available electronic,optical and mechanical elements and provided to the end user as afree-space, collimated beam of broadband light pulses 1.

With a practical example, operation specifications are as follows. Thebroadband core mode spectrum covers a wavelength range from equal to orbelow 250 nm to equal to or above 1100 nm. The average output power ofthe broadband light pulses 1 is above 1 W, and the spectral flatness isbelow 15 dB (between 300 to 1000 nm). FIG. 5 illustrates an example ofan emitted spectrum (calibrated) of the output broadband light pulses 1,ranging from the deep-UV to near-IR. The inset shows a beam crosssection at 1.03 μm.

The features of the disclosure disclosed in the above description, thedrawings, and the claims can be of significance individually, incombination, or sub-combination for the implementation of the disclosurein its different embodiments.

What is claimed is:
 1. A broadband light source comprising: ahollow-core fiber including a filling gas and configured to createbroadband light pulses by an optical non-linear broadening of pump laserpulses, wherein the hollow-core fiber comprises: an axial hollow lightguiding fiber core configured to support core modes of a guided lightfield; and an inner fiber structure surrounding the fiber core andconfigured to support transverse wall modes of the guided light field; apump laser source configured to create and provide the pump laser pulsesat an input side of the hollow-core fiber; and an adjustment devicecoupled to the pump laser source, the adjustment device configured toadjust at least one pump laser pulse parameter.
 2. The broadband lightsource of claim 1, wherein the at least one pump laser pulse parametercomprises a pulse duration, a pulse energy, a pulse shape, a pulsespectrum, a beam modal shape, a beam diameter, a beam polarization, abeam pointing stability, an average beam power, a beam quality, noise,signal-to-noise, and/or a beam power stability.
 3. The broadband lightsource of claim 1, further comprising: a gas supply coupled to theadjustment device and configured to supply the filling gas to thehollow-core fiber, wherein the adjustment device is configured to adjustat least one gas parameter of the filling gas to the hollow-core fiber.4. The broadband light source of claim 1, further comprising amonitoring device coupled to the hollow-core fiber and configured tomonitor at least a part of a core mode spectrum of the broadband lightpulses at an output side of the hollow-core fiber.
 5. The broadbandlight source of claim 4, further comprising a control loop coupled tothe monitoring device and the adjustment device, the control loopconfigured to control the adjustment device based on the core modespectrum.
 6. The broadband light source of claim 5, wherein the controlloop controls the adjustment device such that a spectral displacementbetween the transverse wall modes and the core mode spectrum ismaintained during operation of the broadband light source.
 7. Abroadband light source comprising: an optical head configured to createbroadband light pulses by an optical non-linear broadening of pumppulses, the optical head comprising: a synthesizer including an opticalelement and a hollow-core fiber, the synthesizer configured to couplethe pump pulses to an input side of the hollow-core fiber, wherein thehollow-core fiber comprises a filling gas; a pump source configured tocreate and provide the pump pulses to the synthesizer; a pre-processorconfigured to monitor pump pulse parameters of the pump source; a fluiddevice configured to supply the filling gas to the hollow-core fiber;and a post-processor configured to monitor the broadband light pulses atan output side of the hollow-core fiber; and a control unit coupled tothe optical head and configured to control operation of the opticalhead.
 8. The broadband light source of claim 7, wherein the opticalelement comprises a mirror, a lens, and/or a polarizer.
 9. The broadbandlight source of claim 7, wherein the pre-processor comprises anadjustment device configured to adjust at least one pump pulseparameter.
 10. The broadband light source of claim 7, wherein thepost-processor comprises an adjustment device configured to monitor apart of an emitted spectrum of the broadband light pulses and provide afeedback signal.
 11. The broadband light source of claim 10, wherein thefeedback signal is transmitted to the pre-processor to optimize couplinginto the hollow-core fiber and to the fluid device.
 12. The broadbandlight source of claim 10, wherein the feedback signal is based oninstabilities in the part of the emitted spectrum.
 13. The broadbandlight source of claim 7, wherein the control unit comprises a controlloop configured to automatically regulate one or more parameters of theoptical head to optimize output of the broadband light pulses.
 14. Thebroadband light source of claim 7, wherein the control unit comprises: afirst control sub-unit configured to control the pump source for pumppulse stabilization; a second control sub-unit configured to control thepre-processor and the synthesizer for beam stabilization; and a thirdcontrol sub-unit configured to monitor and to control one or moreparameters of the optical head.
 15. A control system comprising: adetector configured to measure one or more parameters of emittedradiation from a broadband light source comprising a hollow-core fiberto generate measurement data; and a processor configured to performevaluation of mode purity of the emitted radiation based on themeasurement data, wherein, based on the evaluation, the control systemis configured to generate a control signal to optimize one or more pumpcoupling conditions of the broadband light source, the one or more pumpcoupling conditions based on coupling between a pump laser beam and afiber core of the hollow-core fiber.
 16. The control system of claim 15,wherein the one or more parameters of the emitted radiation comprisesone or more parameters of the mode purity of the emitted radiation. 17.The control system of claim 15, wherein the mode purity comprises aratio between a fundamental transverse wall mode power and a totaloutput power.
 18. The control system of claim 15, wherein the detectorcomprises a spectrum analyzer configured to measure one or more spectralparameters of the emitted radiation.
 19. The control system of claim 15,wherein the detector comprises: a bandpass filter configured to filter aspectral range of the emitted radiation; and an optical detectorconfigured to measure a power of the filtered emitted radiation.
 20. Thecontrol system of claim 15, further comprising: an actuator configuredto control movement of one or more components of the broadband lightsource, wherein the control signal is configured to control theactuator.